IP-based mobile systems provide for communication using mobile user equipment on wireless communication networks. Among other elements, the transition of connectivity from one wireless communication system to another would involve a source network and a target network in a transition of communication services. The user equipment, source network, and target network may be called different names depending on the nomenclature used and the base technology used in the particular network configurations or communication systems.
For instance, the term “user equipment” includes a mobile node or mobile communication unit (e.g., mobile terminal, “smart phones”, nomadic devices such as laptop PCs with wireless connectivity, as described in greater detail below), and “user equipment” is sometimes called mobile nodes, mobile unit, mobile terminal, mobile device, or similar names depending on the nomenclature adopted by particular system providers. Direct wireless connectivity to a cellular network supports various makes and models of mobile terminals (“cell phones”) having various features and functionality, such as Internet access, e-mail, messaging services, and the like. But, generally, “user equipment” can encompass PC's having cabled (e.g., telephone line (“twisted pair”), Ethernet cable, optical cable, and so on) connectivity to the wireless network, as well as direct wireless connectivity to the cellular network.
In great majority of deployed mobile networks voice calls are provided over the Circuit Switched (CS) domain. In new mobile network deployments (especially with the advent of 3G High Speed Packet Access (HSPA) and the future E-UTRAN access) it is expected that voice will be supported over the Packet Switched (PS) domain via the IP Multimedia Subsystem (IMS) specified by 3GPP. In order to support voice call continuity between the IMS and the legacy CS domain, 3GPP has initiated specification work on Voice Call Continuity (VCC) in Release-7 specification (TS° 23.206) and this was later generalized to apply to non-voice IMS sessions and was referred to as IMS Session Continuity in Release-8 (TS° 23.237). In the VCC approach for voice continuity, the voice call/session is anchored in a Back-to-Back User Agent (B2BUA) located in the IMS network and referred to as the Session Continuity Control Application Server (SCC AS).
User equipment engaged in a voice call over the PS domain network (in this context the source network) establishes a so-called IMS Access Leg with the SCC AS. If the user equipment is transitioned to a circuit switched communication network (in this context the target network), the circuit switched network establishes a CS Access Leg connection with the SCC AS, which eventually leads to the SCC AS performing the transfer of the voice session on this CS Access Leg connection and releasing of the network resources associated with the IMS Access Leg connection. In order to identify the session that needs to be transferred, however, the SCC AS needs to receive an explicit session identifier. This is typically how the procedure would work in a “dual radio” context, where the CS Access Leg is established by the terminal itself, so it can provide an explicit address identifier for the session that it wishes to transfer.
However, this is not possible in the “single radio” environment (under consideration here), because in single radio environment the CS Access Leg establishment is triggered from the CS domain core network (i.e. the MSC Server enhanced for single radio VCC), rather than from the user equipment. Because the prior art systems (CS domain core network) are unable to provide the explicit identifier of the session that the user wishes to transfer, a new method is needed to allow the SCC AS to identify the session that the user wishes to transfer.